The present invention relates generally to electronic control systems for air conditioners, heat pumps and refrigeration equipment. More particularly, the invention relates to an apparatus and method using digital circuitry for controlling the speed of the indoor fan, based on environmental measurements, to optimize heating and cooling efficiencies and improve occupant comfort. The system modulates the indoor fan speed to stay within the comfort envelope defined by relative humidity and temperature measurements.
The heating and cooling industry has been striving for some time to improve the efficiency of air-conditioning and heat pump systems. System efficiency is a multifaceted concept, with a number of different ways of measuring and evaluating efficiency. Currently in the United States the parameters by which efficiency may be measured and evaluated are established by the Air-Conditioning and Refrigeration Institute (ARI). To allow different makes and models of air-conditioning and heat pump equipment to be compared for efficiency, ASHRAE has promulgated the efficiency measurement standard set forth in ARI 210-81. By those standards, for example, efficiency is measured at the unit's full rated output, i.e., with the unit operating at maximum heating or cooling effort. While this method of rating efficiency does allow units of different manufacturers to be compared on a somewhat common ground, in day to day operation, most heating and cooling equipment cannot be expected to operate continuously at full rated output. In fact, we have found that operating heating and cooling equipment at all times at full rated output actually degrades system efficiency. At many times during the day, and in some regions of the country at nearly all times, optimal efficiency is achieved at well below the maximum rated output. In these regions, energy could actually be saved if the heating and cooling equipment were scaled back to operate below peak output.
To do so, however, is not simply a matter of adjusting the thermostat up or down a few degrees, or scaling back the amount of refrigerant pumped through the system. Heating and cooling systems which have been designed to operate at optimum efficiency during full rated output will not necessarily operate at optimum efficiency when scaled back to less than full rated output. Optimal efficiency is ordinarily fine-tuned into the system during the design and manufacturing process, based on a wide variety of different physical and thermodynamic constraints which are not readily alterable after the system is built. Thus it has heretofore been impractical and uneconomical to provide heating and cooling systems with the ability to optimize efficiency over the normally encountered range of operating limits. Because ARI standards base efficiency ratings on full rated output, heating and cooling systems are understandably designed to provide optimum efficiency at full rated output. In this way the systems will compare favorably with their competition. The result, of course, is that systems, which rarely if ever need to operate at full rated output, rarely or never achieve optimal efficiency.
Compounding the efficiency problem is the issue of comfort. To a considerable extent, relative humidity affects comfort. When conditions are dry one feels comfortable at higher temperatures than when conditions are humid. In humid weather it is often necessary for the building occupant to lower the thermostat below the normal setting just to be comfortable. One benefit of air-conditioning systems is that they remove moisture from the air being cooled.
Surprisingly, however, many highly efficient air-conditioning systems remove less moisture from the air than older, less efficient ones do. This is due to the fact that a highly efficient air-conditioning system nominally operates at higher evaporator coil temperature, which removes less moisture from the air passing over it. The net effect is that sometimes a highly efficient air-condition system ends up consuming more power than an inefficient one, because the user may set the thermostat lower in order to feel comfortable. Present-day air-conditioning systems have not adequately addressed this problem.
The present invention addresses the above efficiency problem in a very effective and inexpensive way. The system employs a variable speed indoor fan which adjusts the airflow across the indoor coil or heat exchanger based on environmental measurements such as outdoor air temperature. The airflow rate across the indoor heat exchanger affects the rate at which heat is extracted (in the cooling mode) or injected (in the heating mode), which in turn affects the superheat within the refrigeration system. The microprocessor-based control system selects the appropriate indoor airflow (fan speed) to place the system in an optimal efficiency range.
The present invention addresses the humidity problem by controlling the speed of the indoor fan to cause a slower airflow when conditions are humid, in order to remove a greater amount of moisture from the air. More specifically, the microprocessor-based control system determines the indoor fan speed to stay within the boundaries of the cooling mode comfort envelope defined by the American Society of Heating and Refrigeration Engineers (ASHRAE), thereby correlating room temperature and humidity.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.